The field of the invention generally relates to voltage regulators, and more particularly, relates to tapped voltage regulators.
As is well known, electronic voltage regulators are used to provide a accurate and steady desired DC output voltage level from a source of higher voltage level that may fluctuate in voltage level over time. Three common types of electronic voltage regulators are the series, shunt, and the switching regulators. In the series regulator a series "pass" element is connected in series with the voltage source. The series pass element is most commonly a transistor which acts as a variable resistance, a variable current source, or a variable voltage source. The output voltage of the series pass element is varied in accordance with a feedback signal. This feedback signal is derived from the desired output voltage, and acts in such a way as to vary the characteristics of the series pass element to keep the desired output voltage at a constant, desired level. With such an arrangement, the desired output voltage remains at a relatively constant level regardless of fluctuations of the source voltage level, or in the impedance of the load. One problem with the series electronic voltage regulator is that its efficiency is relatively low when there is a relatively large differential between the voltage level of the voltage source and the desired output voltage level. For example, if the source voltage level is supplied by a DC battery, the level of which may decrease over time, the initial source voltage level must be relatively high with respect to the desired output voltage level to assure continued proper operation after the source voltage has decreased. However, in the initial operating period, with the relatively high voltage level differential between the source voltage level and the desired output voltage level, power is dissipated in the series pass element at a relatively fast rate thereby reducing the efficiency of the regulator during the initial operating period. This dissipation of power at a relatively fast rate gives rise to the generation within the regulator of heat which must be dissipated.
In the shunt regulator, a shunt "pass" element is connected across, or in parallel with, the voltage source which has a finite output impedance. The shunt pass element is most commonly a Zener diode. The Zener diode acts as a variable resistance which is self-adjusting so as to keep the voltage level across the load equal to the Zener breakdown voltage level by forcing excess source voltage to be dropped across the voltage source output impedance. Therefore, a Zener diode should be chosen which has a Zener voltage equal to the desired output voltage level. Shunt electronic voltage regulators may also have relatively low efficiency during the initial operating period. For example, the efficiency of shunt regulators decreases as the current through the shunt "pass" element increases, as is the case during the initial operating period when the source voltage is substantially greater than the desired output voltage level.
A common switching electronic voltage regulator is a modification of the series electronic voltage regulator, such modification having additional energy storage elements. The difference is that in the switching electronic voltage regulator, the series switching "pass" element, or transistor, is switched between an "on" (low resistance) state, and an "off" (high resistance) state, instead of being set to a variable resistance or a variable current or voltage source somewhere between the aforementioned two extremes. During the "on" state, current flows through the series switching pass element, which has a very small resistance. Because the resistance of the series switching "pass" element is small, very little power is dissipated in the series switching "pass" element. During the "off" state, the resistance of the series switching "pass" element is very high, resulting in a negligible current flowing through the series switching "pass" element. As a result of only negligible current flowing through the series switching "pass" element, very little power is dissipated in it. The desired output voltage level is maintained by a feedback circuit which controls the duty cycle of the series switching "pass" element, or the amount of time the series switching "pass" element is in the "on" state compared to the amount of time it is in the "off" state. The duty cycle is inversely proportional to the difference between the source voltage level and the desired output voltage. That is, as the aforementioned difference increases, the duty cycle decreases; as the aforementioned difference decreases, the duty cycle increases. The pulsed voltage at the output of the series switching "pass element is then filtered to provide the desired output voltage. One problem with the switching voltage regulator is that the peak current through, and in some cases the peak voltage across, the series switching "pass" element is higher than the peak current through, and in some cases the peak voltage across, the series and shunt "pass" elements because the series switching "pass" element must provide the same amount of power in a shorter period of time, i.e., the time during which the series switching "pass" element is "on." Thus, a relatively large and expensive series switching "pass" element is generally required in switching regulators. Further, the corresponding pulses of current created by the switching action have fast rising and falling edges and therefore, high frequency components of current are generated. Such high frequency components may tend to cause high frequency energy to radiate from the switching regulator and cause interference in nearby circuitry.
Another well known method of voltage regulation is provided by a tapped voltage regulator. In such regulator, a plurality of successively increasing voltages is produced at a corresponding plurality of output taps. The desired output voltage is manually selected from one of the plurality of output taps. For example, a plurality of serially connected batteries may be used, each having one of the output taps. With such arrangement, a jumper may be used to physically connect the one of the taps which has a slightly greater voltage than the desired output voltage to the output terminal of the tapped regulator. As the voltage level at the selected tap decreases, the next higher voltage tap is manually connected by the use of the jumper. In this way, there is a small differential between the source voltage and the desired output voltage, as compared with a series voltage regulator, because the selected tap is only slightly higher in voltage than the desired output voltage. However, such arrangement makes a tapped regulator generally unsuitable for use where manual selection of the desired tap voltage is not feasible, such as in systems where the desired tap voltage must be selected more quickly and more accurately than can be done manually, where the cost makes manual selection impractical, or where the desired voltage must be a more exact value than which may be provided by any one of the plurality of taps.